Conventional solid fuel furnaces or combustion apparatus utilize one of four general methods for introducing and burning fuel. These methods are referred to as overfeed firing, underfeed firing, pulverized fuel burning, and fluidized bed combustion. Each of these techniques is very well known and typical examples of types of apparatus employing these techniques are discussed in "Steam Its Generation and Use" by Babcock and Wilcox, 37th Ed. (1963) and "Combustion Engineering" by Combustion Engineering, Inc., Revised Ed. (1966) Lib. of Congress Catalog No. 6623939. The overfeed firing method involves the introduction of fuel into a furnace over the fire in a uniform spreading action such as with a traveling-grate stoker. The overfeed firing method of combustion is relatively inefficient because of the difficulty in achieving complete and even combustion of the fuel and, furthermore, most sulfur-containing fuels require the addition of complex and expensive external scrubbing systems to the furnace. In the case of the underfeed firing method, fuel is introduced into a chamber where a series of pushers or rams move the fuel upward for spreading between air-admitting tuyeres and side-grates. As the fuel rises in the chamber, it is ignited by the heat from the burning fuel above and continues to burn as the incoming raw fuel forces the fuel bed upward. Underfeed firing has many of the same disadvantages as overfeed firing and, furthermore, the ash content must be critically controlled between 3 and 10% so as not to become a hindrance to proper combustion. In pulverized fuel combustion, the fuel is pulverized and then mixed with transport air for conveyance to the furnace where it is burned. Pulverized fuel burning has many drawbacks including the cost of pulverization and the production of large quantities of fly ash which may require the installation of particulate-removal equipment which also adds to the cost of the system. In fluidized bed combustion, the combustible materials are usually ground to a suitable size enabling their fluidization in a stream of high velocity air and combustion takes place in the fluidized bed. The fluidized bed method requires a significant amount of energy to maintain its fluidized state and the temperatures of operation are relatively low. Furthermore, in addition to the fly ash produced, a major disadvantage of this last technique is the incomplete combustion of fines which are swept out of the fluidized bed by the air stream and require either capture and reinjection or burn-out of the carbon in a separate bed.
Other problems are encountered in current state of the art methods of combustion and apparatus. For instance, where a fuel containing sulfur compounds is burned as is the case with coal, the sulfur oxides produced are particularly hazardous to the environment because, upon release into the atmosphere, they combine with water to produce acidic materials, namely, sulfurous and sulfuric acid. When these acids are dissolved in rain, they produce what is commonly referred to as "acid rain" which may cause environmental damage. Sulfur oxides may be removed from the flue gases and most of the methods available involve the treatment of the gases outside the furnace by chemically acceptable means, such as caustic scrubbing or reaction with lime, limestone or dolomite slurries. These methods of sulfur oxide removal require expensive, corrosion-resistive equipment which adds greatly to the cost of the system and requires inconvenient and expensive slurry disposal systems which are environmentally objectionable.
In pulverized fuel burning and fluidized bed combustion systems, powdered or crushed additives such as limestone or dolomite may be added to the fuel for reaction with the sulfur oxides within the furnace. This method is inefficient in systems which burn pulverized fuel because of the relatively high furnace temperatures employed. In fluidized bed combustion, where the temperature is favorable, a substantial part of the sulfur oxides fails to react with the additive and may escape to the atmosphere unless an excess of limestone or dolomite is used. A number of other problems are encountered by employing limestone or dolomite additives in a fluidized bed. For instance, the size of the additive must be controlled such that it is carried out of the system with the fly ash but this does not usually provide optimum reaction time and conditions, or the additive must be of such size as the coal, in which case it becomes coated with calcium sulfate reaction product thereby allowing only a small part of the limestone to react. Where multiple stage beds are employed to overcome this difficulty, high pressure drops usually result across the apparatus with attendant high-energy requirements. Thus, the most common solution to the aforementioned problems is to provide an excess of limestone or dolomite to make up for the unreacted material.
Another disadvantage of known combustion techniques is the formation of nitrogen oxides in the flue gas. These oxides, which form nitric and nitrous acids upon combining with water, cause a major environmental hazard. The formation of nitrogen oxides results from the operation of combustors at relatively high temperatures. Even in the fluidized bed combustor operating at lower temperatures, some nitrogen oxides are produced. While multistage fluidized combustors are being tested in an attempt to reduce the formation of these impurities, as developed above, such combustors involve a high pressure drop across the apparatus with its attendant high-energy requirements.
With the increase in the diversity of materials which need to be burned, combustion apparatus and methods appear to becoming complex. For instance, several hundred billion pounds of refuse are being generated presently each year in the United States alone. The term "refuse" is a term of art which connotes a conglomeration of such diverse materials as cardboard, newspaper, plastic film, leather, molded plastics, rubber, garbage, fluid, stones and metallics, etc. as indicated, for example, in the American Paper Institute Report No. 114, Sept. 11, 1967. Other forms of particulate solids materials or solid-laden gases, sludges, or the like, resulting from municipal sewage sludge, spent foundry sand, refinery sludge, among other waste materials, require disposal. A method for such disposal is incineration. However, government regulations have become very stringent with respect to the types and concentrations of pollutants that may be discharged into our physical environment, virtually prohibiting incineration of waste by many of the heretofore commonly accepted techniques. Similarly, large amounts of convenient open space are no longer available for sanitary land fills and, in any event, communities can no longer tolerate contamination of streams and underground waters from such fills.
Prior art workers have addressed themselves to the problems associated with the combustion or incineration of the above mentioned diverse materials arising from industrial, residential and commercial sources. In addition, it has been an objective for many years to reclaim or recover heat from such waste materials for useful purposes. For example, prior processes have been directed to refuse disposal and heat recovery in steam boilers.
In view of the above brief overview of known methods and apparatus for combustion of materials and utilization of heat therefrom, it is evident that further improvements are needed.